You can get UHD/4K copies if you redeem from some services (including MoviesAnywhere), but you receive HD copies if you redeem from iTunes. Even if you link your iTunes account through MoviesAnywhere, you'll still only receive HD copies from MoviesAnywhere if you redeem the codes through iTunes. I redeemed two of the nine films in iTunes not knowing this, and for those films, I only have access to HD streams in iTunes, MoviesAnywhere, Amazon, and Vudu. I redeemed the other seven films through MoviesAnywhere and have access to 4K copies for those seven films from MoviesAnywhere, Vudu and Amazon (with the strange exception of A New Hope being HD and not 4K from Amazon).
Bottom line: if you want 4K digital copies, don't redeem the codes through iTunes, as you will end up with only HD copies.